Large computer network systems are often logically partitioned into numerous smaller networks, referred to as subnets, virtual local area networks or VLANs. Referring to FIG. 1, it is a block diagram of a network system 100 that has been logically partitioned into three VLANs: VLAN 102, VLAN 110 and VLAN 118. VLAN 102 includes a server 104 and two clients 106 and 108. VLAN 110 includes a server 112 and two clients 114 and 116. VLAN 118 includes a server 120 and two clients 122 and 124. The members of each VLAN communicate with each other through a hub or switch. In the illustrated example, the members of VLANs 102, 110 and 118 communicate with each other through switches 134, 136, and 138, respectively.
All messages sent between members of a given VLAN are sent at the data link layer (level two) of the ISO reference model. Messages sent between VLANs are routed at the network layer (level three) of the ISO reference model. Control information is transmitted with a packet to indicate the device that sent the packet, the device that is to receive the packet, and the protocol-type of the packet. A packet sent between VLANs may include, for example, control information that indicates the protocol-type (PT) of the packet, the data link layer address of the source device (SA), the data link layer address of the destination device (DA), the network layer address of the source device (SNLA), and the network layer address of the destination device (DNLA).
The larger network system 100 is formed by connecting the VLANs to each other through a router 126. Specifically, VLANs 102, 110 and 118 are respectively connected to ports 130, 128 and 132 of router 126. Because the VLANs are connected to each other through router 126, the members of each VLAN are able to communicate with members of other VLANs that are part of the larger network system 100. However, a different mechanism is used to communicate between members of different VLANs than is used to communicate between members of the same VLAN.